Substitutional n-type doping of an organic semiconductor investigated by electron paramagnetic resonance spectroscopy

Doping a perylene diimide organic semiconductor with a one-electron reduced perylene diimide containing a covalently bound counterion provides a well-characterized system for understanding doping in organic semiconductors. We obtain insight into the doping process by electron paramagnetic resonance (EPR) measurements of the dopant solutions, the dopant plus host solutions from which thin films are spin-coated, and the resulting solid films. After correction for some trace impurities in the solutions, the spin density incorporated into the solid films is linearly proportional to the added dopant density. Nevertheless, the film conductivity increases superlinearly with dopant concentration. Although neither pure dopant nor host aggregate in solution, they aggregate when combined. This is presumably a result of the delocalization of the dopant electron over a number of host molecules. Angle-dependent EPR measurements on thin films suggest that the g-tensor symmetry axis is close to the pi-pi stacking axis, consistent with relatively delocalized electrons in this crystal direction. Nevertheless, most electrons are not entirely free, but still bound in the vicinity of the dopant cation by Coulomb attraction. At low concentration, dopants appear to segregate primarily to crystallite grain boundaries, while at higher concentration they are incorporated into the bulk of the crystallites. About half of the spins are paired in the solid at room temperature, and more at lower temperature.